Multi-layer coating systems were developed to satisfy a need for improved aesthetics of the coated substrate. By applying a relatively thin pigmented layer, termed the "basecoat", and then overcoating with a thicker, unpigmented layer, termed the "clearcoat", it was possible to achieve a glossy coating with an appearance of depth that has commonly been called "the wet look".
Multi-layer systems have been utilized to coat automobiles for a number of years, but the early development of these systems employed organic solvents. As environmental regulations became more stringent organic-borne systems became less desirable. The recent research emphasis in the area of multi-layer systems, especially basecoat systems, has focused on the development of water-borne systems for multi-layer coatings.
In particular, as organic-borne systems were formulated to require less and less organic solvent, becoming known in the industry as "high solids" coatings, it became obvious that the appearance, in particular the metallic effect of coatings containing flake pigments, suffered with the increase in solids. One of the opportunities which water-borne systems presents is that of improving on the metallic effect in a coating which complies with the restrictions on volatile organic content (VOC). Water-borne systems, however, resulted in other problems.
The requirements for the rheology of automotive coatings during application and curing steps are rigorous. In the past, rheology control agents have been relied on to provide desired application properties such as prevention of sagging, proper edge coverage, proper orientation of metallic flakes used in the coating, and so on. The range of rheology control agents suitable for automotive water-borne coatings is limited. In many cases the rheology control agents which can be used are difficult to disperse or show poor stability. Some rheology control agents which were successfully used in organic-borne coatings have been adapted for use in water-borne coatings, such as the microgels in the invention of Backhouse, U.S. Pat. No. 4,403,003. But these microgels have the deficiencies of being ardorous to make and difficult to stabilize.
The U.S. Pat. No. 4,822,685 discloses a method of forming a multilayered coating on a substrate, wherein an aqueous polyurethane dispersion is used comprising the reaction product of an isocyanate functional prepolymer and a polyamine. One disadvantage of this method is that even residual amounts of the polyamine in the coatings will cause yellowing and other undesirable effects.
U.S. Pat. No. 4,880,867 discloses an aqueous coating composition comprising a mixture of an acrylic dispersion and a polyurethane dispersion which can be applied by electrostatic spraying. But the acrylic polymer is prepared by solution polymerization at high molecular weights before dispersion. High levels of solvent are required to perform the solution polymerization at a reasonable viscosities. In order to obtain a low VOC coating it would be necessary to distill out the organic solvent that is in excess of what is needed for a stable dispersion. It is also necessary to add a commercial thickener to function as the rheology control agent to obtain the desired appearance of the film.
In addition, in a multilayer coating system it is necessary that the basecoat have "strike in" resistance. By "strike in" resistance is meant the ability of the basecoat to resist attack by the solvents in the topcoat composition. The strike in is a problem because the automobile manufacturers generally wish to apply the topcoat composition in a color plus clear system by a "wet-on-wet" technique. By this is meant that the basecoat composition is applied to the substrate followed by the topcoat composition and then a single baking step is utilized to cure the composite coating. The topcoat "strike in" to the basecoat is particularly undesirable since it adversely affects alignment of the metallic pigment. In highly pigmented colors the non-metallic pigment particles can provide the physical barrier to prevent strike in. But in silver and light metallic colors there is insufficient pigment to prevent strike in. The rheology control agent or a filler pigment must be used to prevent the strike in.
The post-extended anionic polyurethane dispersions of the present invention satisfy the needs for rheology control and strike in resistance in a waterborne coating. In particular, they provide excellent metallic flake orientation in a basecoat or one coat topcoat composition. In addition, they overcome the problems of difficulties of dispersion, instability, deleterious presence of emulsion polymerization surfactants, and high volatile organics content encountered with other compositions; as well as the complicated preparation procedures of sterically stabilized dispersions.
One object of the present invention is to provide an aqueous polyurethane dispersion in a basecoat in an automotive coating system which imparts excellent rheology control and enhances the metallic appearance of a basecoat with flake pigment. Another object of the present invention is to provide a basecoat comprising this aqueous polyurethane dispersion. Yet another object of the present invention is to provide a method of coating a substrate comprising the polyurethane dispersion and a substrate coated therewith.